This invention relates to acoustic velocity well logging apparatus and more particularly to an acoustical logging apparatus having means for interfering with the unwanted acoustical signals which inherently travel in the mud between the transmitter and the receiver of the apparatus during a logging operation.
Acoustical velocity well logging is used to determine certain characteristics of earth formations penetrated by a borehole of a well. Basically, acoustical logging employs a downhole tool which has a transmitter of acoustical signals and a receiver for detecting these acoustical signals after they have traveled through the formation adjacent the tool. Knowing the distance between the transmitter and the receiver, the velocity of the acoustical signal through the formation adjacent the tool is determined by measuring the traveltime of the acoustical signal between the transmitter and the receiver. By correlating the traveltime with the depth at which it occurs, a log is obtained which, when analyzed, provides valuable information as to formations so logged.
Many acoustical logging tools now utilize more than one receiver, wherein the receivers are spaced at greater distances from the transmitter than before. A typical tool of this type spaces these receivers by suspending them from the transmitter on a support member which does not readily transmit acoustic energy, e.g., a cable or the like. The exact distances between the transmitter and the respective receivers are critical in accurately determining traveltimes of the signals, so twisting of the support member as it moves through the borehole should be avoided.
Furthermore, since the borehole is normally filled with a fluid, e.g., drilling mud, during a logging operation, whenever the transmitter emits an acoustical pulse an unwanted signal inherently results in the mud which travels from the transmitter to the receivers. This unwanted signal, if received at approximately the same time as the signal transmitted through the formation, seriously affects the wanted acoustical signal transmitted through the formation.
The undesirability of unwanted signals which travel in the borehole fluid during a similar logging operation was recognized in U.S. Pat. No. 2,350,371 to Smith, issued June 6, 1944, wherein discs of rubber were attached to the support member between the transmitter and the receiver and extended out into contact with the cased borehole to attenuate such signals. However, since normal logging operations are carried out as the tool is raised through a mud-filled borehole, these solid discs, being in contact with the borehole, act as pistons on a pump and seriously impede the raising of the tool. Also, their contact with the borehole may cause the transmitter and receiver to twist relative to each other which, as explained above, is undesirable.